Spa



FIG. 1 is a perspective view of a spa showing our new design;

FIG. 2 is a front elevation view thereof, with the rear, left side and right side elevation views being identical;

FIG. 3 is a top plan view thereof; and,

FIG. 4 is a bottom plan view thereof.

Portions of the spas shown in broken lines depict environmental subject matter only and form no part of the claimed design. 

The ornamental design for a spa, as shown and described. 